properties of fly ash-extended asphalt concrete...

TRANSPORTATION RESEARCH RECORD 1323 123

Properties of Fly Ash-Extended Asphalt Concrete Mixes

ABDULRAHMAN s. AL-SUHAIBANI AND EGON T. TONS

Fillers added to asphalt concrete mixes have long been used as stiffening, extending, or otherwise void-filling materials, depending on their type and particle size . Fly ash, a by-product of the coal-burning process for power generation, is a filler that can serve one or more of the above-mentioned functions. The role of fly ash in asphalt concrete mixes has been investigated. Three sizes of fly ash (coarse, medium, and fine) were used. The effect of these fly ash sizes on the resilient modulus, rutting potential, and water resistance of these mixes was investigated. It was found that the medium-size fly ash was the best size for an asphalt extender.

Fly ash is a finely divided residue that results from furnaceburned pulverized bituminous coal. Electrical generation is the prime consumer of coal produced in the United States and, consequently, the prime producer of fly ash. In the United States, the annual production of fly ash increased from about 43 million tons in 1975 to about 60 million tons in 1979; the utilized amount during that time increased from about 5 million tons, or 10.6 percent of the production, to about 10.0 million tons, or 17.4 percent (1). From these figures, it is clear that there is a huge amount of unused fly ash that must be disposed of each year. This not only costs power companies money but also creates a disposal problem.

Asphalt pavement usually consists of three components: asphalt, aggregates, and air. Because nearly all asphalts used in road construction are of crude oil origin, the increase in crude oil prices in recent years has resulted in an increase in asphalt prices. The dwindling world resources of oil have increased concerns about the asphalt supply for highway pavements. In addition, a considerable portion of asphalt-type highways in the United States are deteriorating as a result of heavy loads and unfavorable weather conditions. All of this has led many researchers and agencies to look for ways of reducing the amount of the required asphalt and improving pavement resistance against loading and moisture damage.

One method that has recently received much attention is the partial substitution, or replacement, of asphalt with other materials. Some mineral fillers have been known to work as integral parts (extenders) of asphalt if thoroughly mixed with it, resulting in an increase in binder content and, therefore, decreasing the amount of asphalt required for optimum conditions.

Fly ash has been used successfully as a filler for asphalt mixes for a long time. It has the advantage of increasing the

A. S. Al-Suhaibani, Civil Engineering Department, College of Enginee~ing, King Saud University , P. 0. Box 800, Riyadh 11421, Saudi Arabia. E. T. Tons, Civil Engineering Department, University of Michigan, Ann Arbor, Mich. 48104.

resistance of asphalt mixes to moisture damage. In addition to filling voids, fly ash reportedly works as an asphalt extender (2). It is the main purpose of this investigation to study the possibility of using wasted fly ash as an asphalt extender in bituminous paving mixes.

LITERATURE REVIEW

As finely pulverized bituminous coal is burned, particles of fly ash are suspended in the gas stream that reaches the boiler. As hot gases pass into the atmosphere, the particles of fly ash collect on the plates of electrostatic precipitators within the heating system. Fly ash is then processed or filtered and finally accumulated and stored (3,4).

Fly ash is characterized by its low specific gravity, which is a function of its chemical composition and varies between 2.3 and 2.6, averaging 2.4 (5). The particle size distribution generally depends on the collector used. It has been found that the ash collected by the electrostatic precipitator contains a greater percentage of very small particles (<1.5 µ) (4). In general, a typical fly ash particle size ranges between 0.5 µ and 100 µ (4). Fly ash particles are generally spherical (4,6); however, a minor fraction consists of irregularly shaped particles ( 6).

Most types of fly ash have been used successfully as mineral fillers in hot asphalt mixes ( 4-6 ). It has been reported that fly ash does not differ materially from Trinidad Asphalt's mineral filler (7). In another study, fly ash was used as a replacement for limestone dust in asphalt concrete mixes with good results (8). The suitability of fly ash as a filler in sheet asphalt mixes was investigated by the Detroit Edison Company and reported by Zimmer (9). It was found that stabilities of mixes containing fly ash are comparable to those containing limestone dust when proportions are based on weight, and that fly ash has virtually the same void-reducing properties as limestone dust.

The resistance of asphalt mixes containing fly ash to water action has been found to be equal or superior to those containing other types of filler (6,10). The strength retention for eight fly ashes ranges between 85 and 100 percent-all above the 75 percent figure considered to be the critical minimum.

A report by Rosner et al. indicates that the addition of up to 6 percent fly ash by weight of aggregate to asphalt concrete produces an acceptable mix (11). At the same time, asphalt requirements and voids in mineral aggregates (VMA) values are lower than those for mixes containing portland cement or hydrated lime.

Tons et al. investigated the use of fly ash as a replacement for asphalt cement in asphalt concrete mixes (2). The three

124

types of fly ash used in the study were found to have positive effects on the physical properties of the evaluated mixes . Furthermore, the replacement of up to 30 percent of asphalt cement by fly ash improved most of the mix's physical properties when a practical asphalt content was used.

Concerning other types of fillers, Kallas and Puzinauskas found that 9 of the 11 fillers investigated were effective in replacing a portion of the asphalt required to produce minimum VMA (12). Other reports also point out that some mineral fillers can serve as asphalt extenders, and that they may actually decrease the stiffness of the asphalt (13,14). Baghouse fines, also , were reported to act as asphalt extenders (15-18) .

Finally, it has been found that mixing fine fly ash particles (passing Sieve #32S) with asphalt cement causes the highest increase in viscosity compared with coarser fly ash sizes (19) .

STUDY OBJECTIVES

This investigation had two objectives:

1. To study the effect of fly ash particle size , aggregate gradation, and binder content on the resilient modulus and rut-depth characteristics of asphalt concrete mixes .

2. To evaluate the use of fly ash as an asphalt extender in asphalt concrete mixes.

EXPERIMENTAL WORK

Materials

Asphalt Cement

AC-20 asphalt cement was used for this study. It was obtained from Marathon Petroleum Company in Detroit, Michigan. The results of tests conducted on the asphalt are shown in Table 1.

TRANSPORTATION RESEARCH RECORD 1323

Fly Ash

Fly ash used in this study was obtained from Consumer's Power Company, Michigan. The fly ash was dry-sieved on Sieves #270 and #32S for lS min. Because fly ash has practically no particles finer than 1 µ, silica fume (microsilica), which is 100 percent finer than O.S µ, was mixed with the fly ash fraction passing Sieve #32S in a SO/SO ratio by weight to form the fine fraction. This was done to study the stiffening effect of very fine materials . Table 2 summarizes the specific gravities and particle size distribution of the three fractions .

Aggregate

The coarse aggregate was a crushed gravel obtained from Thompson-McCully Asphalt Paving Company in Whitmore Lake, Michigan. The fine aggregate was a concrete sand obtained locally.

The coarse aggregate was sieved and divided into different sizes as follows: 3/4 in .- V2 in ., V2 in.-3/s in., :Ys in.-#4, #4-#8. Each size was then thoroughly washed , dried , and stored until usage. The fine aggregate was divided into the-following sizes: #8-#16, #16-#30, #30-#SO, #S0-#100, #100-#200.

The coarse and fine aggregate fractions were combined with the desired proportions according to the gradation curves A, B, and C. The three gradations were chosen in such a way that the sand content was different for each one; however, they were all within the specification limits of the Michigan Department of Transportation. The fine gradation (C) represents the finest, and the coarse gradation (A) represents the coarsest. The medium gradation (B) falls between the two. An important property of these aggregate gradations is that they possess different surface areas . This is important because the larger the surface area of aggregates, the larger the amount of asphalt needed to coat such aggregates. The three gradation curves are shown in Figure 1, and their specific gravities and absorptions are shown in Table 3.

TABLE 1 SUMMARY OF ASPHALT CEMENT PROPERTIES

Test

Specific Gravity (77/77° F)

Viscosity x 106 ,

poises (77° F)

Penetration (77° F) 0 .1 lllill, 100 g' 5 sec

Penetration (95° F) 0.1 mm, 100 g, 5 sec

Softening Point, of (R & B)

P.I.

* For Recovered Asphalt

ASTM Test Designation

D 70-76

D 3570-77

D 5-73

D 5-73

D 36-76

Test Results

1. 022

1.504

74

(143)*

122 (129)*

(-0.7)*

Al-Suhaibani and Tons 125

TABLE 2 PROPERTIES OF FLY ASH FRACTIONS

Grain Size Analysis Fly Ash Fraction Coarse Medium Fine*

Sieve size: # 30 (595 µ.) 99.9 100 100 # 100 (144 µ.) 93 100 100 # 200 ( 75 µ.) 64 100 100 # 270 ( 54 µ.) 54 100 100

Hydrometer Analysis

# 325 (44 µ.) 47 100 100 30 36 94 97 20 25 79 90 13 18 54 77

9 13 41 70 7 10 30 65

3.5 4 10 55 1. 5 0 2 51

1 0 0 50 0.5 0 0 50 0.2 0 0 48 0.1 0 0 35 0.084 0 0 25

.05 0 0 10

.01 0 0 2

Specific Gravity 2.012 2.383 2.259

50 percent by weight silica fume

en c:

"' "' .. Q.

... c:

" u .. " Q.

100

I I 80 Aggregole Grodollon

60 -

40

20

0 .0 1

----·--· .............. A B c

I/ / .. ,

JP I.' " .I

·' ' II

·" Ill 1' .... •· 1//

/ l/ •' v ,

,,.

Sieve Size, mm

v .•

v

10

FIGURE 1 Particle size distribution of aggregates.

Test Variables and Specimens Preparation

100

A set of variables was chosen for this study. In addition to the fly ash sizes and aggregate gradations already mentioned, two other variables-asphalt content and percentage of replacement-were used.

Because asphalt content is not constant in the replacement process, the term "asphalt equivalent," which represents the asphalt content for control mixes, was used instead. Three asphalt equivalents were used in this study-namely, 4, 5, and 6 percent. The asphalt cement at each asphalt equivalent was replaced by an equal volume of fly ash in five replacement percentages: 0, 10, 20, 30, and 40 percent. The asphalt cement was partially replaced by fly ash so that the total volume of binder (asphalt + fly ash) was kept constant and equal to the original asphalt volume before the addition of fly ash.

All specimens used were Marshall size and were cast according to ASTM standards with 50 blows on each side of the specimen.

Either one or two specimens were prepared for each mix, as shown in Table 4. For control mixes (no fly ash), two specimens were prepared. This experimental design was used to reduce the quantity of materials needed and the time necessary for preparing and testing specimens.

CHARACTERIZATION OF MIXES

Resilient Modulus

Resilient modulus test before and after immersion was conducted on the same specimen; the nondestructive nature of the test made this possible.

Specimens were immersed in water at 140°F for 24 hr. Testing for resilient modulus was then done for the purpose of evaluating the effect of test variables (mainly fly ash size and quantity) on resistance of mixes to water action. By testing the same specimen before and after immersion, errors owing to material variability were eliminated. At the same time, a reduction in materials' quantities, time, and effort was achieved.

It should be mentioned that although only samples of the results are reported here, the conclusions were based on the results of the entire study (19).

Dry Resilient Modulus

Dry resilient modulus results for 6 percent asphalt equivalent are shown in Figure 2. These results show that, in general and for a given aggregate gradation and binder content, fine fly ash causes the most stiffening of the three fly ash fractions. This is consistent with the results of viscosity testing of asphalt-fly ash mixes (19), which show that the fine fly ash causes the highest stiffening effect among the three fly ash sizes. Medium fly ash ranks second; coarse fly ash gave the

126 TRANSPORTATION RESEARCH RECORD 1323

TABLE 3 SPECIFIC GRAVITIES AND ABSORPTIONS OF AGGREGATE GRADATIONS

Parameter Aggregate Gradation A B C

Bulk Sp. Gr. 2.599 2.600 2.600

Bulk Sp. Gr. (SSD) 2.645 2.643 2.640

Apparent Sp. Gr. 2.725 2.717 2.709

Percent Absorption 1. 775 1.662 1. 554

TABLE 4 MIX VARIABLES AND NUMBER OF SPECIMENS

Aggregate Gradation

A 8 c Fly Ash Size Fly Ash Size Fly Ash Size

F M c

A!i!llllillt -l::!ll! inli!Dt 4 Percent of Asphalt Replaced

0 percent 2 2 2 10 percent 1 2 1 20 percent 2 1 2 30 percent 1 2 1 40 percent 2 1 2

Asllhalt -l::m!illilli!nt ::!

Percent of Asphalt Replaced

0 percent 2 2 2 10 percent 2 1 2 20 percent 1 2 1 30 percent 2 1 2 40 percent 1 2 1

AllllllS!lt -fimiival§!!lt 6


0 percent 2 2 2 10 percent 1 2 1 20 percent 2 1 2 30 percent 1 2 l 40 percent 2 1 2

lowest modulus values. This kind of behavior is expected because the fine fraction (contains an appreciable amount of submicron particles), when combined with asphalt, caused its viscosity to increase to values higher than those caused by the other two fractions. This translates into a high resilient modulus in asphalt-fly ash concrete mixes. As for coarse fly ash, given its large particle size, it creates more voids in the mix, hence producing a less stiff mix than the other two.

For a given aggregate gradation and asphalt equivalent, dry resilient modulus increases as the percent of replacement increases for fine and medium fly ash, but it remains the same or decreases for coarse fly ash. The increase in the resilient modulus in the case of fine fly ash appears to be logical, given its high stiffening effect. The ineffectiveness or the negative effect of coarse fly ash is mainly attributable to the increase

F M c F M c

2 2 2 2 2 2 2 1 2 1 2 1 1 2 1 2 1 2 2 1 2 1 2 1 1 2 1 2 1 2

2 2 2 2 2 2 1 2 1 2 1 2 2 1 2 1 2 1 1 2 1 2 1 2 2 1 2 1 2 1

2 2 2 2 2 2 2 1 2 1 2 l l 2 l 2 1 2 :. l 2 l 2 1 1 2 1 2 1 2

in voids in mixes made with this fraction as its quantity increases.

For a given aggregate gradation, increasing asphalt equivalent from 4 to 6 percent has little or no effect on dry resilient modulus. The details of this effect can be found elsewhere (19).

In general, for a certain asphalt equivalent, changing the aggregate gradation from A to C shifted the resilient modulus values downward. This was more pronounced for mixes containing coarse fly ash and mixes with low replacement percentages. Grain interlock, which is more pronounced for Gradation A, could be the reason for this behavior.

Estimates of the main effects of the four independent variables on dry resilient modulus are shown in Table 5. These effects seem to strengthen the previous conclusions. These

Al-Suhaibani and Tons

3

2

Aggregate Gradation : A

Fl y Ash Size Fine

--•-- Medium --·-•--· Coarse

o+---.-~~~--..~~~--r~-.---.----1

., o . .Y.

"' . 0

x

'" ::::J

::::J -0 0

I: ..., c: Q)

'" Q)

a:

0

3

2

10 20 30

Aggregate Gradation :

Fly Ash Size --m-- Fine

Medium --·-•-·- Coarse

40 50

>'Cl o+---.-~~~--..~~~--r~-.---.----1

0 10 20 30 40 50 5..--~~~~~~~~~~~~--.

Aggregate Gradation: C

Fly Ash Size -<1-- Fine -- •-- Medium •• •• .,.... Coarse

OT--~~~~--.~-.---.-~~-.---..--1

0 10 20 30 40


FIGURE 2 Dry resilient modulus versus percentage of asphalt replaced for 6 percent A.E.

50

effects are represented by ex, (3, "/, and A. in the prediction model shown below.

Yijklm = µ + CX; + f3j + "lk + Ai + Eijklm (1)

where

Y,iklm = the predicted value of the dependent variable for a given set of levels of the independent variables,

µ = the grand mean (the constant in Table 5), a, = estimated main effect of the ith level of the asphalt

equivalent, (3i = estimated main effect of the jth level of the ag

gregate gradation, "lk = estimated main effect of the kth level of the per

centage of asphalt replaced,

127

f..1 = estimated main effect of the lth level of fly ash size, and

E;iklm = an error term.

These parameters indicate the effect of each level of a given independent variable on the predicted values. A plus sign indicates a positive effect; a minus sign indicates a negative effect. A very low value (approximately zero) indicates little or no effect. In other words, the estimated main effects show the contribution of each level of the independent variable toward the predicted values . Table 5 also shows the standard deviation of these main effects.

Immersion Effect

To study the effect of water on asphalt-fly ash concrete mixes, the index of retained stiffness (IRS) was used. IRS is the ratio of resilient modulus after and before water immersion. It indicates the resistance of a given mix to water action.

Figure 3 shows a sample of the results of soaked resilient modulus . Corresponding IRS values are shown in Figure 4. Tables 6 and 7 show the statistically estimated main effects for soaked resilient modulus and IRS, respectively. These results show that, in general, IRS value gets lower as the aggregate gets finer and as the asphalt equivalent gets lower. However, much lower IRS values were obtained for mixes made with Gradation A and fine fly ash at 40 percent replacement. In fact, all mixes made with fine fly ash at 40 percent replacement show lower IRS values than those mixes made with medium or coarse fly ashes. This was expected, because these mixes were not completely coated with asphalt, allowing water to easily strip asphalt films from aggregate surface, thus reducing IRS values below those of other mixes .

Rut Depth Prediction

One of the major distresses that occur in bituminous pavement is permanent deformation, or rutting. Permanent deformation has received considerable attention from researchers and pavement technologists. Several methods have been introduced in pavement design to address the problem of rutting. These methods are divided into two main categories: indirect and predictive (20). Indirect methods are based on limiting layer thickness and component material strengths, stability, or density to a minimum, or limiting the vertical compressive strain on the subgrade surface to some maximum level. Predictive methods are based on predicting rut depth for a given set of conditions and redesigning the pavement if necessary until a satisfactory rut depth value is obtained.

A predictive method that relates pavement permanent deformation to creep test results was developed by Shell Oil Company Laboratories. Because of its simplicity as a tool for comparing different mixes and the availability of the needed equipment to run the creep test, this method has been adopted here to evaluate the rutting potential of asphalt-fly ash concrete mixes (21).

To predict rut depth, the static creep te~t-as suggested by Shell-was conducted on Marshall specimens representing


TABLE 5 ESTIMATED MAIN EFFECTS FOR DRY RESILIENT MODULUS x 106 (kPa)

Parameter

Constant

Asphalt Equivalent

6 Percent 5 Percent 4 Percent

Aggregate Gradation

A B c


10 Percent 20 Percent 30 Percent 40 Percent

Fly Ash Size

Fine Medium Coarse

different mixes. Each mix was represented by either one or two specimens (see Table 4), which were made and tested at random. The test was performed for 1 hr in a temperaturecontrolled chamber maintained at about 104°F (40°C). The time-dependent deformation was measured using a pair of linear variable differential transformers (L VDTs) that were connected to a computer to take readings, calculate the mix stiffness, and store the data. Time-dependent mix stiffness (Sm;.) was later plotted in a log-log scale against the stiffness of recovered asphalt (Sb;,). The scale, obtained from a Van der Poe! nomograph (22), was for the same conditions as those of Sm;x·

For rut depth calculation, pavement must be divided into layers and sublayers; then, for a given air temperature, the asphalt effective viscosity is calculated for each sublayer. Effective viscosity, the slope of the Sm;x-Sb;, relationship, and traffic data were used thereafter to calculate the viscous part of the asphalt stiffness (Sb;" vise). This value was then used to obtain the viscous part of the mix stiffness from the Sm;xSb;, relationship.

The rut depth was then calculated using the following equation:

where

ilh; = rut depth in the ith sublayer, ilh = total rut depth,

(2)

(3)

Estimated Main Effects

l. 88

0.02 0.03

-0.06

0.36 0.09

-0.45

-0.48 -0.19 0.10 0.57

0.53 0.10

-0.63

Standard Deviation

0.03

0.06 0.06 0.06

0.06 0 . 06 0.06

o. -'.l6 0 . 06 0.06 0 . 06

0.06 0.12 0.06

cm = correction factor for dynamic effect, h; = sublayer thickness,

smix,i = mix stiffness for the ith sublayer' obtained from the Sm;x-Sb;, relationship, and

Z; proportionality factor between average stress and contact stress.

The factor Z in the equation is a function of mix stiffness in the high-stiffness region (short loading time), the sublayer thickness, the stiffness and the thickness of other sublayers, the modulus and thickness of the base course, and the subgrade modulus. The values of Z were calculated using the elastic layer theory and tabulated for different combinations of the previously mentioned variables.

The expected rut depth in each mix was calculated for the conditions shown in Table 8.

The results of the calculated rut depth are plotted in Figures 5 and 6. It is worth mentioning that most mixes made with Gradation C failed under the static load, some after only a few minutes of loading, so no data were obtained for thse mixes. As the results show, the general trend for all mixes containing fine and medium fly ash fractions is that the rut depth decreases as the replacement increases. Mixes made with Gradation A and coarse fly ash show similar results. However, mixes made with Gradation B and coarse fly ash show an increase in rut depth as the percent of replacement increases. The reason that the coarse fly ash increases the rut depth as the replacement increases for Gradation B and not A is probably that the aggregates' interlocking-and hence the mix's resistance to loading-is reduced by increasing the percentage of replacement in the case of Gradation B, which contains a larger amount of sand. On the other hand, Gradation A contains a small amount of sand; hence, even with


'1

3

2

0

"' 0 0..

5

"' ' 0 '1

x

"' ;:, 3 ;:,

"' 0 I: ~ 2 c .,

"' Q)

a:

"' Q)

0 "" " 0 0 (J) 5

'1

3

2

Aggregate Gradalion: A

Fly Ash Size ---<>-- f ine --• - - Medium ·-· ·•·- · Coarse

10 20 30

Aggregate Gradalion :

Fly Ash Size --e- fine --·-- Medium ........... Coarse

'10

B

~----· ----____ ....

_ .... ~:"'-~-- -·····-M·· · -· · ········· ···-....

10 20 30 '10

Aggregate Gradalion: c

Fly Ash Size

- fine --·-- Medium ............. Coarse

........................................ ,,:

50

50

o+-~~--~-.--~--.~....--.--~-1

0 10 20 30 '10


FIGURE 3 Soaked resilient modulus versus percentage of asphalt replaced for 6 percent A.E.

50

the addition of coar e fly ash, the interlocking among large aggregate particles-and, in turn, the rut depth - is not much affected by the replacement process.

For a given aggregate gradation and asphalt equivalent fine, medium, and coarse fly a h fractions gave the lowest , intermediate, and highest rut depth , respectively when compared at the same percentage of replacemenl However there are a few exceptions. Ranking mixes containing the tb1·ee fly ash fractions in the order mentioned is expected given the stiffening effect of each one as was shown in the discussion of resilient modulus results.

The rut depth results for the three fly ash fraction are generally the same in the case of Gradation A but not B. This could be auributed to difference in residual asphalt lmit weight and air void between mixes made with Gradation A and those made with B.

When mixes are compared at the same aggregate gradation, the result indicate, although not very obviou ly, that increa -

1.2

1.0

0.8

0.6

0.'1

0.2 0

1.2

"' "' Cl> c

1.0 .... .... ~

Cf)

'C 0.B Q)

c

2 ., 0.6 a:

.... 0

x 0.'1 Cl>

"' c

0.2 0

1.2

1.0

O.B

0.6

0.'1

0.2 0

Aggregate Gradation : A

Fly Ash Size

10

fine Medium Coarse

20 30

Aggregate Gradation: B

~-:--~.:.:~~-~~::.">-::_-:·x;. ~-

Fly Ash Size

fine

--·-- Medium Coarse

10 20 30

Aggregate Gradation: C

Fly Ash Size

Fine

Medium

·~··· - - Coarse

10 20 30

'10 50

'10 50

'10 50


FIGURE 4 Index of retained tiffness versus percentage of asphalt replaced for 6 percent A.E.

129

ing asphalt equjvalent from 4 to 6 percent increases rut depth for control mixes (no fly a h added) and steepens lopes for curves representing the Tut depth ver u percentage of replacement. The fir t observation could be explained as follows: for control mixes the residual a phalt is more at high than that at low asphalt equivalent. Con equently the trengtb of bigh asphalt equivalent depend more on visco ity (resi -tance to flow) of the asphalt film than on particle-to-particle contact. This results in weaker mixes and, consequently, higher rut value in case of mixes made with hjgh asphalt equivalent. The as ociation of steeper slopes with higher a phalt equivalent could be attributed to the fact that , at a certain percentage of replacement the higher the asphalt equivalent, the higher the amount of fly ash tliat should be added to the mix to replace asphalt at a given percentage of replacement. This, in turn produces mixes With lower asphalt/aggregate ratios, resulting in stiffer mixes as the percentage of replacement increases in ca e of high asphalt equivalent.

TABLE 6 ESTIMATED MAIN EFFECTS FOR SOAKED RESILIENT MODULUS x 106 (kPa)

Parameter

Constant

Asphalt Equivalent


Aggregate Gradation

A B c


10 Percent 20 Percent JO Percent 40 Percent

Fly Ash Size

Fine Medium coarse


1. 24

0.17 0.01

-0.16

0.17 0.13

-o. 30

-0.17 -0.05

0.17 -0.04

0.26 0.10

-0.36

TABLE 7 ESTIMATED MAIN EFFECTS FOR INDEX OF RETAINED STIFFNESS

Parameter

Constant

Asphalt Equivalent


Aggregate Gradation

A B c



Fly Ash Size

Fine Medium Coarse


0.69

0.08 -0 . 0J -0.05

-0.03 0.02 0.01

0.08 0.06 0.02

-0.16

-0.02 0.001 0.02

standard Deviation

0.03

0.03 0.03 0 . 03

0.03 0.03 O.OJ

0.05 0.05 0.05 0.05

0 . 03 0.03 0.03

standard Deviation

0 . 008

0.01 0.01 0.01

0.01 0.01 0.01

0.01 0 . 01 0.01 0.01

C. 01 0 . 01 0 . 01

I


TABLE 8 CONDITIONS FOR CAJ:,-CULATING EXPECTED RUT DEPTH (23)

Condition

Asphalt-fly ash concrete layer thickness Sublayer-1 thickness Sublayer-2 thickness Sublayer-3 thickness Unbound base course thickness Subgrade modulus Axle loads/lane/day Design period Traffic growth/year Ann Arbor weather data for 1984

Asphall Equiva lenl: 63

E E

:5

6

5

4

3 0

7

6

5

Fly Ash Size - Fine Medium Coarse

10 20 30

Asphall Equivalenl: 5?.

...... ,, ' , ' ,, ' ,, ' ,, ' ......... ... ............. _ ............. ',

40

0.. Q)

0 ........ ----· ....... ···-·-··•· 1C

~

::> 0: 4

Fly Ash Size

- Fine -- .. -- Medium · ··· •·· · Coarse

Measurement

180mm 40mm 40 mm

100 mm 300 mm

108 N/m2

2,000 15 years 2 percent

50

3+--..-~.--~---.~~-.-~~..,.-~...---1

0 10 20 30 40 50 7~~~~~~~~~~~~~~,

6

5

4

Asphall Equivalenl: 47.

Fly Ash Size ----e-- Fine - - .. - - Medium

Coarse

3-1-~~~~~~~~~~~~~--1

0 10 20 30 40 50 Percenl of Asphall Replaced

FIGURE 5 Estimated rut depth versus percentage of asphalt replaced for Aggregate A.

E E

:5 0.. Q)

0

Asphalt Equivalenl : 67. ---·« .... -······

.... ---6 '~:· ··· •••• .•••.•. -·····-

\

5

4

3 0

7

\ \

\

'v ........

Fly Ash Size

- Fine --·-- Medium Coarse

10 20 30 40

Asphalt Equivalenl: 57. ... JC ••

6 ....••. /· ···• ......... .

5

4

----"" ' ' ' ' ' ' ,

Fly Ash Size --e- Fine

'It''

-- .. -- Medium -·--•··- Coarse

.. •• • M

...... .,. .. ,,-···

50

3+-~~..--~---.~~-.-~~..,.-~---1

0 10 20 30 40 50 7 ..-~~~~~~~~~~~~----,

6

5

4

Asphall Equivalent: 47.

Fly Ash Size

Fine -- .. - - Medium ····•··· Coarse

3+-~~..-~~..-~~..-~~~~~

0 10 20 30 40 50 Percent of Asphalt Replaced

FIGURE 6 Estimated rut depth versus percentage of asphalt replaced for Aggregate B.

131

For 4 and 6 percent asphalt equivalents, the use of Gradation B resulted in a higher rut depth for control mixes than those produced by Gradation A. However, for 5 percent asphalt equivalent, the rut depth is about the same for both gradations. It was expected that control mixes made with Gradation B should give higher rut depth than those made with A (as is the case with 4 and 6 percent asphalt equivalent) , because A produced mixes having higher unit weight and lower air voids than those made with B. So, the behavior of mixes made with 5 percent asphalt equivalent could only be attributed to experimental error.

For a given asphalt equivalent, changing aggregate gradation from A to B resulted in an increase in rut depth for mixes containing coarse fly ash. This could be explained by both unit weight and air voids, because mixes containing Gradation A possess both lower air voids and higher unit weight than those containing Gradation B (19).


TABLE 9 ESTIMATED MAIN EFFECTS FOR ESTIMATED RUT DEPTH (mm)

Parameter

constant

Asphalt Equivalent


Aggregate Gradation

A B c



Fly Ash Size

Fine Medium coarse

Table 9 shows the estimated main effects of various variables on the estimated rut depth. Also shown are the standard deviations of these effects. These effects are consistent with those mentioned earlier. A descriptive model of these effects was presented earlier.

CONCLUSIONS

It was the purpose of this study to investigate the feasibility of using fly ash as an asphalt extender and to study the effect of fly ash particle size and other variables on the replacement process. The following i a ·ummary of the main conclu ions:

1. Fly ash having particle sizes between 1 and 44 µ is the best as an asphalt extender because coarser particles tend to create more voids among aggregate particles and finer ones tend to stiffen mixes. In the latter case, a mix with low workability is produced that also possesses large amount of voids after compaction.

2. It was strongly indicated that increasing sand content increases the sensitivity of rutting potential of different mixes to changes in sizes of fly a h particles.

3. For a given asphalt equivalent and aggregate gradation, the three fly ash fractions are very similar in their effectiveness against moisttue damage except for mixes having 40 percent replacement.

4. At 40 percent replacement , the fine fraction caused the least resistance to moisture damage.

5. As far as this study shows, it is reasonable to replace up to 40 percent of asphalt volume with medium fly ash (1 through 44 µ in ize) for dry climates; however, for moi t climates, the replacement should not exceed 30 percent.


5,32

0.053 -0.046 -0.007

-0.50 0.16

-0.66

0.35 0.27

-0.20 -0.42

-0.53 0.15 0.38

standard Deviation

0.07

0.09 0.09 0.09

0.09 0.99 0.99

0.12 0.10 0.10 0.10

0.09 0.09 0.09

PRACTICAL APPLICATIONS

The results of various tests performed on different mixes clearly show that the replacement of up to 30 percent of the asphalt volume by medium-size fly ash (pa ·sing Sieve #325) proved to be not detrimental to the mixes' performance. Becau e 80 percent of mo ·t fly ash produced by power plants pa e Sieve #325, fly a h should be quite ati factory when u ed to replace up to 30 percent of the asphalt volume. lt i also expected that the e mixe should perform well if they are in tailed in the field. Furthermore, substituting fly ash for part of the asphalt can mean savings for the road builders, fewer disposal problems for the fly ash producers, and les damage to the environment.

REFERENCES

1. Summary on Use of Fly Ash in Construction. GAI Consultant, Nov. 1983.

2. E. Tons ct al. Fly Ash as Asphalt Reducer i11 Bi111mi11011s Base Courses. Universiiy of Michigan Ann Arbor, June 1983.

3. W. L. Chilcote. Fly A h: It Possibilities as a Source of Fines in Portland Cement Concrete and Bituminous Concrete l>avement Mixtures. U. S. Navy Civil Engineers Corps Bulletin, Vol. 6, Oct. 1952, pp. 279-281 .

4. S. Torrey. Coal Ash Utilization: Fly Ash , Bottom Ash , and Slag. Poll111io11 Technology Review, No. 48, 1978.

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6. J. P. Capp and J . D . Spencer. Fly Ash U1ilizatio11: A w1111rary of Applicatio11s a/Ill Technology. Information Circular 8483. Bureau of Mines, U.S. Department of the Interior 1970.

7. C. M. Weinheimer. Evaluating fmportancc of the Phy ical and Chemical Properties of Fly Ash in Creating Commercial Outlets


for the Material. Transaction of the American Society of Mechanical Engineers, Vol. 66, No . 6, Aug. 1944, pp. 551-561.

8. L. J . Minnick . New Fly Ash and Boiler Slag Uses . Technical Association of the Pulp and Paper Industry , Vol. 2, No . 1, Jan. 1949, pp. 21-28.

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10. C. A. Carpenter. A Cooperative Study of Fillers in Asphaltic Concrete . Public Roads, Vol. 27 , No . 5, Dec. 1952, pp. 101-110.

11. J . C. Rosner et al. Fly Ash as a Mineral Filler and Anti-Strip Agent for Asphalt Concrete . Proc., 6th International Ash Utilization Symposium, Vol. 1, July 1982, pp. 57-78 .

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14. E. L. Dukatz and D. A. Anderson. The Effect of Various Fillers on the Mechanical Behavior of Asphalt and Asphaltic Concrete . Proc., Association of Asphalt Paving Technologists , Vol. 49, 1980, pp. 530-549.

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133

16. G. W. Maupin, Jr. Effect of Baghouse Fines on Compaction of Bituminous Concrete. Report VHTRC 81-R49. Virginia Transportation Research Council, Charlottesville , May 1981.

17. D . A . Anderson and J . P. Tarris. NCHRP Report 252: Adding Dust Collector Fines to Asphalt Paving Mixtures. TRB, National Research Council, Washington, D.C., Dec. 1982.

18. D. A . Anderson et al. Dust Collector Fines and Their Influence on Mixtures Design. Proc., Association of Asphalt Paving Technologists , Vol. 51, 1982, pp. 363-397.

19. A. S. Suhaibani. The Use of Fly Ash as an Asphalt Extender in Asphalt Concrete Mixes . Ph.D. dissertation. University of Michigan, Ann Arbor, 1986.

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21. Shell Pavement Design Manual-Asphalt Pavements and Overlays for Road Traffic. Shell International Petroleum Company Ltd., London, England, 1978.

22. C. Van der Poet. A General System Describing the Visco-Elastic Properties of Bitumens and Its Relation to Routine Test Data. Journal of Applied Chemistry, Vol. 4, May 1954, pp. 221-236.

23. Climatological Data, Michigan. Vol. 89, No . 1-12, 1984.

Publication of this paper sponsored by Committee on Nonbituminous Components of Bituminous Paving Mixtures.

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